1. Field of the Invention
The present invention relates to an improved DC (direct current) to DC converter.
2. Description of the Related Art
DC to DC converters each with a transformer are capable of isolating between input and output thereof so that they have been widely used. In transformer DC to DC converters, various types of them each having two transformers have been well-known.
For one example, Japanese Unexamined Patent Publication No. 2003-102175 discloses a DC to DC converter system including two DC to DC converters. Each of the DC to DC converters has a substantially conventional structure with one transformer. The two DC to DC converters are connected to each other in parallel and they perform complementary operations. In the disclosed DC to DC converter system, alternately current outputs of one and the other transformers allow ripple components to decrease by only lower-capacitance smoothing capacitors without using any choke coils in the output.
For another example, U.S. Pat. No. 5,291,382 discloses a DC to DC converter system having two transformers.
In the structure of the DC to DC converter system related to the disclosure, as shown in FIG. 32, a DC voltage is fed from a DC power supply 102 to the primary winding W101 of a transformer T100 and a primary winding W102 of a transformer T200 through a switching element Q101. As the switching element Q101, a MOS (Metal Oxide Semiconductor) transistor may be used.
The primary windings W102 and W105 are connected in series, and they are connected to a clamp circuit. The clamp circuit is composed of a capacitor C102 and a switching element Q102, such as a MOS transistor, connected to each other in series. The reference characters “D” represent intrinsic diodes of the switching elements Q101 and Q102, respectively. Secondary windings W103 and W106 are connected to each other in series. Voltages applied to the secondary windings W103 and W106 of the transformers T100 and T200 are alternately rectified by a conventional synchronous rectifying circuit 100 to be outputted as output voltages Vo.
The switching elements Q101 to Q104 are PWM (Pulse-Width Modulation) controlled to regulate the output voltages Vo. The switching elements Q101 and Q102 are turned on and off alternately (complementarily).
Operations of the DC to DC converter illustrated in FIG. 32 will be briefly described hereinafter.
(First Mode)
When the switching element Q101 is turned on, the DC voltage is applied to each of the first primary windings W102 and W105 while the switching element Q102 is off state. The DC voltage causes the current i101 to pass from the input terminal through the primary windings W102 and W105 and the capacitor (input smoothing capacitor) C101, which is connected to the DC power supply 102 in parallel, discharges in the discharging direction DD shown in FIG. 32.
The inductances L of the primary windings W102 and W105 cause the current i passing therethrough to ramp up with time, so that voltages are generated in the secondary windings W103 and W106. The polarities of the generated voltages are positive at their dot side terminals, respectively.
When the switching element Q103 is turned on, the current i103 based on the generated voltage in the secondary winding W106 is outputted therefrom, and the magnetic energy based on the generated voltage in the secondary winding W103 is stored in the core of the transformer T100.
(Second Mode)
When the switching element Q101 is turned off, the stored magnetic energy in the transformer T100 causes the voltage at the connecting point 140 between the non dot-side terminal of the secondary winding W105 and the connecting point connecting between the switching elements Q101 and Q102 to increase rapidly. The rapidly increased voltage at the connecting point 140 causes the capacitor C102 to be charged in the charging direction CD through the intrinsic diode D of the switching element Q102.
(Third Mode)
When the switching element Q102 is turned on, the magnetic energy in the transformer T100 causes the capacitor C102 to be effectively charged through the switching element Q102 so that the magnetic energy in the transformer T100 is reduced. After the charge of the capacitor C102 based on the magnetic energy has been completed, the stored voltage in the capacitor C102 causes a current to flow through the switching element Q102 to the primary windings W105 and W102 in the discharge direction illustrated in FIG. 32. That is, the capacitor C102 discharges in the discharging direction DD. The discharged current ramps up with time so that voltages are generated in the primary windings W105 and W102. The polarities of the generated voltages are positive at their non dot-side terminals, respectively.
When the switching element Q104 is turned on, the current i104 based on the generated voltage in the secondary winding W103 is outputted therefrom, and the magnetic energy based on the generated voltage in the secondary winding W106 is stored in the core of the transformer T200.
(Fourth Mode)
When the switching element Q102 is turned off, the stored magnetic energy in the transformer T102 causes the voltage at the connecting point 140 to decrease rapidly. As a result, for discharging the magnetic energy generated in the second transformer T200, the current i102 passes through the input terminal and the intrinsic diode D of the switching element Q101, so that the capacitor C101 is charged in the charging direction CD.
When the switching element Q101 is turned on, the current based on the stored energy in the transformer T200 flows into the capacitor C101 so that the capacitor C101 is charged. After the charging operation from the transformer T200 in the capacitor C101, the operation cycle consisting of first to fourth modes is terminated, returning to the first mode. That is, the operation cycle (First mode to Fourth mode) is repeated.
The structure of the DC to DC converter system illustrated in FIG. 32, however, permits the reverse current to flow out from the transformer T200 toward the input DC power supply 102, which may increase ripple components contained in the input current outputted from the DC power supply 102 and inputted to the transformers T100 and T200.
This may require to reduce the voltage range of the DC power supply 102, thereby reducing the ripple of the input current.
From this requirement, in the DC to DC voltage converter system, a large capacitance capacitor, which is connected in parallel to the input DC power supply 102 as the capacitor (input smoothing capacitor) C101, may be used. The large capacitance capacitor C101 is of large size and expensive, causing the DC to DC voltage converter system to be upsized and the cost thereof to increase.
In addition, the increased ripple components contained in the input current passing through the line connecting between the DC power supply 102 and the DC to DC converter into the transformers T100 and T200 may cause the line to radiate electromagnetic waves, which may require an electromagnetic shield for shielding the electromagnetic waves. The increased ripple components may also increase an effective value of the input current, which may result in increasing loss of energy and heating value of the DC to DC converter system.
Moreover, in the DC to DC converter system illustrated in FIG. 32, the direct current outputted from the DC power supply 102 is inputted to the transformers T100 and T200. This direct current inputted each of the transformers T100 and T200 bias the magnetization of each of the cores, which may cause each transformer to be upsized to suppress the bias magnetic field of each core.